Add --install-dir arg to uv python install and uninstall (#7920)

## Summary

This PR adds `--install-dir` argument for the following commands:
- `uv python install`
- `uv python uninstall`

The `UV_PYTHON_INSTALL_DIR` env variable can be used to set it
(previously it was also used internally).

Any more commands we would want to add this to? 

## Test Plan

For now just manual test (works on my machine hehe)

```
❯ ./target/debug/uv python install --install-dir /tmp/pythons 3.8.12
Searching for Python versions matching: Python 3.8.12
Installed Python 3.8.12 in 4.31s
 + cpython-3.8.12-linux-x86_64-gnu
❯ /tmp/pythons/cpython-3.8.12-linux-x86_64-gnu/bin/python --help
usage: /tmp/pythons/cpython-3.8.12-linux-x86_64-gnu/bin/python [option] ... [-c cmd | -m mod | file | -] [arg] ...
```

Open to add some tests after the initial feedback.

---------

Co-authored-by: Zanie Blue <contact@zanie.dev>

crates/uv-cli/src/lib.rs

@@ -4250,6 +4250,16 @@ pub struct PythonDirArgs {
 #[derive(Args)]
 #[allow(clippy::struct_excessive_bools)]
 pub struct PythonInstallArgs {
+    /// The directory to store the Python installation in.
+    ///
+    /// If provided, `UV_PYTHON_INSTALL_DIR` will need to be set for subsequent operations for
+    /// uv to discover the Python installation.
+    ///
+    /// See `uv python dir` to view the current Python installation directory. Defaults to
+    /// `~/.local/share/uv/python`.
+    #[arg(long, short, env = "UV_PYTHON_INSTALL_DIR")]
+    pub install_dir: Option<PathBuf>,
+
     /// The Python version(s) to install.
     ///
     /// If not provided, the requested Python version(s) will be read from the
@@ -4310,6 +4320,10 @@ pub struct PythonInstallArgs {
 #[derive(Args)]
 #[allow(clippy::struct_excessive_bools)]
 pub struct PythonUninstallArgs {
+    /// The directory where the Python was installed.
+    #[arg(long, short, env = "UV_PYTHON_INSTALL_DIR")]
+    pub install_dir: Option<PathBuf>,
+
     /// The Python version(s) to uninstall.
     ///
     /// See `uv help python` to view supported request formats.